Personally I would not mix battery ages in the same bank. Ideally all batteries that are parallel for discharging should be the same size, age, and brand.
AGM batteries are not a good value for most deep cycle applications (look at fork trucks, golf carts, etc, all flooded). As mentioned, 2 or 4 GC2 duracell/deka brand flooded batteries will cost you less than 110$ each, and provide excellent service. They will need the water checked every 90 days or so, and an equalize charge every month ideally.
DO NOT trust the AGM/flooded/etc settings on any charger. You must physically verify the voltages and absorb times. The majority of consumer grade chargers go to float much to early. It takes hours at the absorb voltage to fully charge a lead battery. Failing to get a full 100% charge when cycling a battery will dramatically cut its lifespan.
Many AGM and flooded batteries can be charged with the same absorb voltage. Review the MFGs specs to determine. With the GC2 batteries above, you can use the Trojan GC2 recommendations, as they are a good match (just much better value).